DENVER — The biggest loser in this year’s Colorado election cycle may have been New York City Mayor Michael R. Bloomberg.

Colorado voters kicked the mayor to the curb at the ballot box again, rejecting a proposed statewide income-tax hike backed by Mr. Bloomberg just two months after ushering out two Democratic state senators who supported his gun-control agenda.

Mr. Bloomberg sunk $1 million into the $10 million campaign for Amendment 66, a proposed $1 billion tax increase to fund K-12 education. Despite an enormous spending advantage — opponents of the ballot measure only raised about $20,000 — Amendment 66 lost by a whopping 66 percent to 34 percent.

The lopsided defeat comes after Mr. Bloomberg was unable to save Democratic state Sens. Angela Giron and John Morse in the Sept. 10 recall election, even though both Democrats enjoyed a huge funding edge, thanks in part to a $350,000 donation from the mayor.

To top it all off, voters in the town of Telluride, Colo., doused a proposal Tuesday to add an excise tax to sales of sugary soda pop. After gun control, the mayor may be best known for his fight to limit consumption of sugary soft drinks.

Even Gov. John Hickenlooper, a Democrat, hinted last month that the mayor’s involvement may have been hurting more than helping, telling USA Today that Coloradans “don’t really like outside organizations meddling in their affairs.”

Foes of the tax increase were in full I-told-you-so mode Wednesday after election returns showed Mr. Bloomberg suffering another ignominious defeat.

“Bloomberg is now 0 for 3 in Colorado in 2013. When will this guy learn to just stay away?” said Morse recall organizer Rob Harris in a message on Twitter.

Referring to both Mr. Bloomberg and Microsoft founder Bill Gates, who chipped in another $1 million to the tax-hike campaign, Mr. Cadman added that he hoped the next legislative session would reflect “the will of the people — I mean those who actually live here.”

Colorado has become an irresistible testing ground for out-of-state political players promoting their issues because of the state’s small size, purple-state credentials and relatively inexpensive media markets. The population is just 5 million, and the Denver media market is ranked 17th in the nation and Colorado Springs-Pueblo is ranked 90th.

“Mayor Michael Bloomberg should think about a different post-mayor career. Investing in Colorado elections is not only a losing proposition, but his name and money is counterproductive,” Denver pollster Floyd Ciruli said in a Wednesday blog post.

“And while Amendment 66 was ill-positioned from its inception, Bloomberg did not help and possibly reinforced, if not initiated, some late ‘no’ votes,” Mr. Ciruli said. “Move on Michael, move on.”